This invention relates to belt grinders in general and more particularly to an improved belt grinder.
Several designs of belt grinders for chip board and the like, including: an endless grinding belt which revolves in a longitudinal plane perpendicular to a feed direction for the board and extends over the width of the board; a counterpressure element parallel to the pressure roll, the pressure roll and counterpressure element together defining a grinding gap through which the grinding belt is conducted; and a feeding device for the board, advancing in the feed direction, are known.
In a first design, the grinding belt is looped around the pressure roll. A support roll which is parallel to the pressure roll and forms the counterpressure element, is provided in the region of the looping angle. The board is pushed through the gap between the two rolls which forms the grinding gap.
In another design, the grinding belt runs over a grinding beam, adjacent to which may be deflection rolls which are arranged on both sides at the same height. In this case, the grinding beam forms the counterpressure element. The pressure roll is arranged above the grinding beam. The grinding gap is formed between the pressure roll and the top side of the grinding beam.
In order to bring about a satisfactory grinding operation with economical operating speeds, a certain line pressure on the order of about 104 N/m is required in the grinding gap. The line pressure must, of course, be very uniform over the width of the sheet, because otherwise the removal is very nonuniform and the board has a poor thickness tolerance. The thickness tolerances are very critical, especially in the further processing of the sheets on automatic veneering, laminating and varnishing machines. The requirements extend here to a tolerance of +0.1 mm. To maintain such tolerances is no easy task, especially for sheet widths of up to about 2.5 m, because the pressure rolls are already bent appreciably under the line pressure at such lengths, so that the sheets have the tendency to become thicker in the center. To eliminate this influence, the pressure rolls must have a relatively large diameter.
It is an object of the present invention to design a belt grinder of the type described above in such a manner that narrow tolerances of the thickness of the ground boards are possible without excessively increasing the diameter of the pressure rolls.